Nanotechnology Spotlight

An evolutionary tree for nanotechnology particle engineering

(Nanowerk Spotlight) Many of the properties of a given nanoparticle not only depend on its chemical composition but also on its size and shape, i.e. its morphology. These morphological factors have significant impact on a nanoparticle's optical and catalytic properties. Accordingly, nanoparticle manufacturers have developed numerous 'recipes' for synthesizing particles with desired size and shape.

"To facilitate systematic investigation on the morphology-property relationship, it would be highly desirable if one reaction system can be engineered to yield as many different shapes as possible with minimal degree of parameter tuning," Jiaxing Huang explains to Nanowerk. "To that end, we proposed a way to systematically engineer the morphologies of nanoparticles by constructing an evolutionary tree, which consists of several pathways, each showing a 'string' of evolving shapes over the courses of a single reaction. The tree not only displays the relationship between different shapes, but also offers designing principles for producing more complex shapes by crossing over different pathways during nanoparticle growth."

Huang, an assistant professor at Northwestern University's Department of Materials Science & Engineering, and his group were motivated by how morphologies evolve in nature: "In nature, growth of species is usually accompanied by evolutionary changes in morphology over time until a final steady state is reached, like human beings ourselves," he says. "If you only watch our steady state adult 'morphology', you would have missed a lot of exciting moments during our growth. When we first discovered that gold nanorods can transform into several different final shapes, we started to get curious about how they evolve over time. That eventually led the idea of developing an evolutionary tree."

Setting up an evolutionary tree offers a more systematic view for shape control of nanoparticles. This way one can display how the shape of particle evolves over time.

Using nanorods as seed, Huang's team successfully constructed an evolutionary tree of gold nanoparticle growth consisting
of three independent branches. He points out that, instead of making one final shape, a single chemical reaction is capable of producing a set of shapes. The exact shape produced is determined by the reaction progress, which can be easily controlled by the reaction time or the amount of reactants.

Evolutionary tree of gold nanorod overgrowth consisting of three branches. Each pathway carries a unique set of codes guiding the morphological evolution. Crossing over two evolutionary pathways can create 'hybrid' morphologies carrying both sets of codes. (Image: Ahyoung Kim, Kwonnam Sohn and
Franklin Kim, Northwestern University)

Exploring the growth mechanisms of gold nanoparticles, Huang's team first discovered that multiple independent evolutionary
pathways could be established starting from the same seed particle using the same reaction system. In each pathway, the seeds
can evolve through a set of intermediate states as the reaction progresses until a steady state shape is reached, after which the particles only grow in size.

Huang explains that each pathway carries a unique set of 'codes' guiding the morphological transformation such as the growth direction and/or the preferred surface crystallographic orientation of the final shape.

"Therefore, instead of producing a single final product, each reaction readily yields a string of continuously tunable sizes and shapes without changing any reaction parameters."

This insight allowed the team to construct an evolutionary tree that displays a library of nanoparticles grown from the same seed. The tree also offers ground rules for designing new shapes. Almost reminiscent of Gregor Mendel's experiments with pea plants, this work also shows that crossing over different pathways can generate new morphologies carrying the codes of both branches.

"Since the optical property of a gold nanoparticle depends on its size and shape, it continuously changes during the course of reaction until a near steady state (stable final state) is reached," says Huang. "Therefore, the tree can tell you where to stop along the reaction progress if you want a specific set of optical properties, such at what wavelength (color) should the nanoparticles absorb or scatter most."

He hopes that the concept of an evolutionary tree in nanoparticle synthesis will offer inspiration towards morphology engineering of nanoparticles of other materials. It may ultimately lead to the realization of on-demand nanoparticle synthesis based on desired final properties.

The researchers assume that their current three-branch tree is very likely only a portion of the crown in a much bigger evolutionary tree originating from the universal ancestor – the gold precursor chloroauric acid. Huang notes that the completion of such a comprehensive tree and the construction of evolutionary trees for other reaction systems or even different materials should eventually lead to the rational "total synthesis" of nanoparticles.

A particular challenge turns out to be the uniformity of starting seeds. Gold nanorods happen to be one of the most studied nanomaterials, which can now be made in large quantities with relatively good uniformity.

"We would like to extend this to other materials, starting from other metals such as silver. Ultimately we would like to perform morphological control of nanoparticle at a level as sophisticated as organic total synthesis, where complex molecules (such as taxol, a cancer fighting drug) can be constructed step-by-step to achieve the final functionality."